Vehicle safety air bags are folded inside a housing mounted on the hub of the steering wheel in front of the driver or on the dashboard in front of the right front seat passenger. The front wall of the cover of the housing is designed to break when the air bag is inflated by providing weakened zones, such as grooves or perforations that define fracture lines along which the front wall fractures under the forces applied to it by the inflating air bag. The strength of the weakened zones has conventionally been uniform throughout. Accordingly, the location of the point of initial fracture can vary, depending largely on the way in which the air bag is folded. For example, the fracture may start at a location near the end of a fracture line, i.e., near the edge of the front wall. Also, at low temperatures, the front wall may break in unexpected ways. The way in which the fracture of the cover occurs can affect the way in which the air bag inflates, in that the shape of the fully or nearly fully inflated bag may not be optimal.